The present invention relates to an automotive anti-lock brake control system for preventing locking of the wheels of an automobile when the automobile is braked.
FIG. 2 of the accompanying drawings illustrates a conventional automotive anti-lock brake control system. In the illustrated earlier automotive anti-lock brake control system, a master cylinder 101 is connected to front and rear wheel cylinders 102, 103 by pipes 104, 105, respectively, which have three-position directional control valves 106, 107, respectively. The three-position directional control valves 106, 107 are of a known structure having three selectable positions, i.e., a communicating position in which the master cylinder 101 communicates with the wheel cylinders 102, 103, a cutoff or neutral position in which the master cylinder 101 is out of communication with the wheel cylinders 102, 103, and a return position in which the wheel cylinders 102, 103 communicate with a reservoir 108. Between the three-position directional control valve 107 and the rear wheel cylinder 103, there is provided a known proportional valve 109 for developing a hydraulic pressure difference between the hydraulic pressure supplied to the front wheel cylinder 102 and the hydraulic pressure supplied to the rear wheel cylinder 103. The hydraulic brake fluid is pumped from the reservoir 108 by a hydraulic pump 110 and the hydraulic pressure from the pump 110 is stored in an accumulator 111.
When the anti-lock brake control system starts to operate, the three-position directional control valves 106, 107 are switched between the three positions for changing the hydraulic pressure in the wheel cylinders 102, 103 thereby to prevent the wheels from being locked. Since these directional control valves 106, 107 switch very quickly, large pressure variations or pulsations are repeated in those portions of the pipes which are located downstream of the valves 106, 107 with respect to the direction of fluid flow from the master cylinder 101. The proportional valve 109 is supplied with the brake fluid pressure (input pressure) from the directional control valve 107 and supplies the rear wheel cylinder 103 with an optimum brake fluid pressure (output pressure) according to the input fluid pressure applied to the proportional valve 109. When the fluid pressure downstream of the directional control valve 107, e.g., at a position P1, is subjected to repeated frequent variations, the proportional valve 109 is also caused to repeat its proportioning operation (described above) as frequently. As the number of cycles the proportional valve 109 undergoes is increased, its durability and hence service life tend to be reduced relatively soon.
As shown in FIG. 3, the proportional valve 109 develops a pressure hysteresis characteristic between its input pressure at the position P1 (FIG. 2) and its output pressure at a position P2. Therefore, even after the directional control valve 107 is shifted to the return position and the input pressure is reduced, the output pressure is not reduced immediately but remains as it is for a certain period of time. This leads to a problem in that the timing for lowering the pressure in the wheel cylinder 103 is delayed.